1. Field of the Invention
The present invention provides a catalyst system for the copolymerization of olefins, specifically ethylene and propylene, that include the combination of a conventional supported Ziegler-Natta catalyst with an electron donor compound of the formula: 
wherein R1 is a linear alkyl group attached to the silicon atom; R2 and R3 are alkyl or aryl groups and R4 is a linear alkyl attached to the silicon atom, R1 and R4 are the same or different.
2. Description of the Prior Art
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst, a co-catalyst, usually an organoaluminum compound; and an electron donor compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos. 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of catalyst systems designed primarily for the polymerization of propylene and ethylene.
A conventional supported Ziegler-Natta catalyst for the copolymerization of olefins is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. These catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum.
An electron donor compound is used in the polymerization reaction to reduce the atactic form of the polymer thereby giving control of and increasing the production of isotactic polymers. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or group of compounds with which it is especially compatible. It has been shown that different dimethoxy donors can be used to affect catalyst performance. The structure of the donor alkyl groups has been shown to affect catalyst hydrogen response. Where the donor contains two linear (i.e., unbranched) alkyl groups, the hydrogen response of the catalyst is enhanced in comparison to donors having branched groups, e.g., cyclohexylmethyldimethoxysilane (CMDS).
The production of high melt flow index ethylene/propylene copolymers requires high levels of hydrogen in the reactor. In the case of a bulk process, the hydrogen is normally completely dissolved in the propylene liquid, but an increase in hydrogen beyond the saturation point would result in the formation of a gas pocket (gas-cap) in the reactor. This results in poor circulation of the reactor and production problems. Moreover, if hydrogen cannot be increased, the donor level may be reduced to increase the effectiveness of the hydrogen, but this would also cause an increase in xylene solubles.
A need has been recognized for higher melt flow index (MFI) copolymer resins. Two obvious methods of increasing the copolymer MFI are: (1) raising the reactor hydrogen level, and (2) lowering the donor concentration to increase hydrogen response. However, these approaches are not practical since the reactor hydrogen level is near saturation, that is, near reactor gas-capping conditions, and the xylene solubles are at the upper level of desirability. For example, if the hydrogen used in a loop reactor process for the copolymerization of propylene in liquid propylene to produce high melt flow index copolymer resins is 0.7 mole percent, and saturation occurs at 0.73 mole percent (145xc2x0 F., 510 psig). Under these conditions-reduction of donor concentration increases xylene solubles, which tend to be already high in the target range. One solution would be to produce copolymer having a melt flow index of about 1-29 and visbreaking the copolymer at extrusion with an organic peroxide which degrades at high temperature. However, this solution adds another process step, is difficult to control and can add undesirable properties, such as yellowing. The production of high MFI copolymers using d-n-butyldimethoxysilane compared with a standard donor, such as cyclohexylmethyldimethoxysilane (CMDS) donor was examined.
The present invention provides a process for the copolymerization of olefins, specifically, ethylene and propylene, using the combination of a conventional supported Ziegler-Natta type of catalyst component with a specific group of electron donor compounds which results in significant increases in improved control of the properties of the copolymer product, specifically melt flow index. The catalyst is preferably a Ziegler-Natta catalyst comprising a complex of a dialkoxy magnesium support and a titanium halide, as disclosed in U.S. Pat. Nos. 4,927,797; 4,816,433 and 4,839,321. The electron donor a silane compound of the following formula: 
wherein R1 is a linear alkyl group attached to the silicon atom; R2 and R3 are alkyl or aryl groups and R4 is a linear alkyl attached to the silicon atom, R1 and R4 are the same or different, R2 and R3 are the same or different. In preferred embodiments of the invention, R1 is a linear alkyl group of 4-13 carbon atoms, more preferably, 4-7 carbon atoms, R2 and R3 are methyl, ethyl or propyl groups, and R4 is a linear alkyl group of 4-13 carbon atoms, more preferably 4-7 atoms. R1 and R4 are preferably the same and, most preferably, are n-butyl. R2 and R4 are preferably the same and, most preferably, are methyl. The most preferred electron donor is di-n-butyldimethoxysilane (DBDS).
In addition, the system may contain an organoaluminum compound which acts as a co-catalyst. The organoaluminum co-catalyst is preferably an aluminum alkyl, more preferably a trialkylaluminum of the formula AlR{circumflex over ( )}3 where R{circumflex over ( )} is an alkyl having 1-8 carbon atoms, R{circumflex over ( )} being the same or different. Examples of trialkylaluminums are trimethyl aluminum (TMA), triethyl aluminum (TEAl) and triisobutyl aluminum (TiBAl). A preferred co-catalyst is triethylaluminum (TEAl).
The process for the copolymerization of olefins comprises: contacting the new generation polymerization catalyst with an organoaluminum compound, preferably with a trialkylaluminum; contacting the catalyst with an electron donor either simultaneously with or after contact with the organoaluminum, the electron donor being a silane compound as described by the formula above, pre-polymerizing the catalyst by contacting a small amount of monomer with the catalyst; and introducing the catalyst into a polymerization reaction zone containing the organoaluminum compound, the electron donor and the monomers. The process further comprises withdrawing directly from the reactor a copolymer product in which the melt flow index is from about 15 to about 60 without further processing to modify melt flow properties.